Centrifugal Compressor

ABSTRACT

There is provided a centrifugal compressor having a diffuser structure in which airflow is unlikely to separate from a hub side wall surface on a hub side wall surface downstream side within a diffuser passage. In a centrifugal compressor provided with a diffuser passage  15  for recovering static pressure by slowing down discharged air from an outer circumferential end of an impeller that rotates within a housing  11 , a hub side wall surface  15   b  of the diffuser passage  15  is provided with an inclined plane  20  that approaches toward a shroud side, in a position on a downstream side of a portion parallel with a normal line direction of a section surface of an impeller exit.

TECHNICAL FIELD

The present invention relates to a centrifugal compressor of a turbocharger or the like.

BACKGROUND ART

Conventionally, a centrifugal compressor of a turbo charger or the liketo be used for an internal combustion engine of a motor vehicle forexample, is commonly known.

FIG. 5A is a sectional view showing a relevant section of a conventionalcentrifugal compressor. A centrifugal compressor 10 shown in the drawingcompresses fluid such as gas and air introduced from outside a housing11, by rotating an impeller 13 having a large number of blades 12 withinthe housing 11. The fluid flow (airflow) formed in this way travelsthrough an impeller exit (Hereunder, also referred to as “diffuserentry”) 14, being an outer circumferential end of the impeller 13, thenthrough a diffuser passage 15 and a scroll 16, and is then delivered tothe outside. Reference symbol 17 in the drawing denotes a shaft axisabout which the impeller 13 rotates.

The diffuser passage 15 mentioned above is provided in between theimpeller exit 14 and the scroll 16, being a passage for recoveringstatic pressure by slowing down the airflow discharged from the impeller14. The diffuser passage 15 is normally formed from a pair of opposingwall surfaces. In the description below, one of the pair of the opposingwall surfaces is called a shroud side wall surface 15 a and the other iscalled a hub side wall surface 15 b.

In a turbo charger of a motor vehicle to be used in combination with aninternal combustion engine, since a wide compressor operation range isrequired, a type of diffuser that does not have vanes (vanelessdiffuser) is normally employed.

Incidentally, in recent years, in response to an increase in flow amountand pressure ratio of the centrifugal compressor 10, distortion of theairflow flowing into the diffuser passage 15 is becoming greater. It isthought that a flow with a large distortion flowing into the diffuserpassage 15 results in the occurrence of a phenomenon known as surging,which determines the operation limit on the small flow amount side inthe diffuser passage 15.

In the mechanism that gives rise to surging it is thought that surgingoccurs when a backflow area of the airflow reaches an exit side endsection of the diffuser passage 15. Moreover, since the occurrence ofthe backflow area mentioned above is thought to be caused by a flow onthe compressor shroud side within the diffuser passage 15, that is,distortion of the flow along the shroud side wall surface 15 a, adiffuser structure for reducing such distortion in the flow is proposed.

The distortion in the flow mentioned above refers to the flow velocitydistribution or pressure distribution being in a non-uniform state.Conventional techniques for making this uniform employ a structure ormethod that changes a flow passage sectional area of the diffuserpassage 15, or that utilizes a circulation passage or the like. Each ofsuch conventional techniques has a focus on reducing distortion thatoccurs on the entry side (impeller exit 14 side) of the diffuser passage15.

As a conventional technique for reducing the distortion mentioned above,for example, provision of a convex section or a concave section forchanging the flow passage sectional area on the wall surface of thediffuser passage 15 has been proposed. Such a convex section or concavesection regularize the airflow along the circumferential direction bychanging the passage shape in the circumferential direction, therebyenabling an improvement in compression efficiency. (for example, referto Patent Document 1).

Patent Document 1: Japanese Unexamined Patent Application, PublicationNo. Hei 10-176699

DISCLOSURE OF INVENTION

However, in a small centrifugal compressor such as a turbo charger for amotor vehicle in particular, it is difficult to measure the internalflow of the airflow flowing within the diffuser passage. Therefore,since distortion in actual internal flow has yet to be sufficientlyunderstood and furthermore the phenomena up to the point of surging havenot been explained, it is necessary to understand these and develop aneffective surging prevention method to widen the operation range of thecentrifugal compressor.

As shown in FIG. 5B, the conventional diffuser passage 15 generally hasa shape in which a pair of opposing wall surfaces, namely a shroud sidewall surface 15 a and a hub side wall surface 15 b, are in a parallelform, so that an axial direction flow passage width W of the diffuserpassage 15 is constant in the circumferential direction. Having examineda flow pattern of up to surging, by carrying out internal flowmeasurement using a model for the conventional diffuser passage 15configured in this way, it was revealed that in the downstream area ofthe backflow area (shown with an arrow A in the drawing) becomes a flowarea that reaches the diffuser exit 18 side end section of the diffuserpassage 15, the airflow separates from the hub side wall surface 15 b inthe vicinity of the diffuser exit 18, forming a backflow area (shownwith an arrow B in the drawing). That is to say, it is thought that itis not a shroud side backflow area A, but a backflow area B which causessurging to occur.

It is thought that such a flow separation from the hub side wall surface15 b occurring in the vicinity of the diffuser exit 18 cannot besuppressed by a conventional technique, as disclosed in Patent Document1 for example, in which airflow is regularized along the circumferentialdirection by changing the passage shape in the circumferentialdirection.

In consideration of the above circumstances, an object of the presentinvention is to provide a centrifugal compressor having a diffuserstructure in which airflow is unlikely to separate from a hub side wallsurface on the hub side wall surface downstream side within the diffuserpassage (in the vicinity of the diffuser exit).

The present invention employs the following means to solve the aboveproblems.

A centrifugal compressor according to the present invention is acentrifugal compressor provided with a diffuser passage for recoveringstatic pressure by slowing down discharged air from an outercircumferential end of an impeller that rotates within a housing,wherein a hub side wall surface of the diffuser passage is provided withan inclined area that approaches toward a shroud side, in a position ona downstream side of a portion parallel with a normal line direction ofa section surface of an impeller exit.

According to such a centrifugal compressor, since the hub side wallsurface of the diffuser passage is provided with the inclined area thatapproaches the shroud side, in the position on the downstream side ofthe portion parallel with the normal line direction of the sectionsurface of the impeller exit, the radial direction velocity of the lowvelocity area that occurs on the hub side wall surface increases in theinclined area, and the radial direction velocity distribution within thediffuser passage is made uniform.

The inclined area in this case is a portion of an inclined plane, acurved surface, or a stepped plane, formed on the hub side wall surfacefor example, and must be such that, in this inclined area, the axialdirection flow passage width of the diffuser passage narrows down fromthe upstream side toward the downstream side by inclining the hub sidewall surface so as to approach toward the opposed shroud side wallsurface as it approaches the downstream side.

In the above aspect of the invention, a preferable position in thediffuser passage length direction for providing the inclined area, inthe diffuser passage in a range between the diffuser entry taken as abase point (0) and the diffuser exit (1), is within a range on thedownstream side (exit side) having a ratio of 0.3 to 0.7.

Moreover, in the above aspect of the invention, it is preferable thatthe inclined area provided in the diffuser passage be such that themaximum value of an amount of projection from the hub side wall surfacetoward the shroud side wall surface is set to approximately ⅓ to ⅕ ofthe passage width, being equivalent to the size of the measured backflowarea.

Furthermore, in the above aspect of the invention, the preferableinclination angle is less than or equal to 20 degrees based on thenormal line of the impeller exit section when the inclined area is takenas a plane. However, a more preferable inclination angle is between 2degrees and 10 degrees, inclusive, based on the normal line of theimpeller exit section. An excessively large inclination angle is notpreferable, as the airflow is re-accelerated due to the reduction in thepassage area in this case.

According to the present invention described above, since on the hubside wall surface of the diffuser passage there is provided the inclinedarea that approaches the shroud side in the position on the downstreamside of the portion parallel with the normal line of the impeller exitsection, the radial direction velocity of a low velocity area thatoccurs on the hub side wall surface increases in this inclined area.Therefore, the radial direction velocity distribution within thediffuser passage is made uniform, and separation of the airflow isunlikely to occur locally, so that the surge flow rate can be reducedand the compressor operation range can be widened.

Moreover, the present invention described above is suited for wideningthe operation range of a small centrifugal compressor provided with avaneless diffuser, such as a turbo charger for a motor vehicle, whichrequires a particularly wide compressor operation range.

BRIEF DESCRIPTION OF DRAWINGS

[FIG. 1] is a sectional view of a diffuser passage showing a firstembodiment of a centrifugal compressor according to the presentinvention.

[FIG. 2] is a sectional view showing a first modified example of FIG. 1.

[FIG. 3] is a sectional view showing a second modified example of FIG.1.

[FIG. 4] is a sectional view of a diffuser passage showing a secondembodiment of a centrifugal compressor according to the presentinvention.

[FIG. 5A] is a sectional view showing a relevant section of thecentrifugal compressor.

[FIG. 5B] is a sectional view showing a conventional structure of adiffuser passage.

EXPLANATION OF REFERENCE SIGNS

-   10: Centrifugal compressor-   11: Housing-   12: Impeller-   14: Impeller exit (diffuser entry)-   15, 30: Diffuser passage-   15 a, 30 a: Shroud side wall surface-   15 b, 30 b: Hub side wall surface-   16: Scroll-   18: Diffuser exit-   20: Inclined plane (inclined area)-   21: Inclined curved surface (inclined area)-   22: Inclined inflected line (inclined area)-   33: Shroud side inclined section (inclined area)

BEST MODE FOR CARRYING OUT THE INVENTION

Hereunder an embodiment of a centrifugal compressor according to thepresent invention is described, based on the drawings.

As shown in FIG. 5A, a centrifugal compressor 10 is provided with adiffuser passage 15 that recovers static pressure by reducing thevelocity of a discharged air from an outer circumferential end of animpeller 13 that rotates within a housing 11. The diffuser passage 15 isprovided so as to connect between an impeller exit (diffuser entry) 14and a scroll 16, and is formed in between a pair of opposing wallsurfaces that comprise a shroud side wall surface 15 a and a hub sidewall surface 15 b.

FIG. 1 is a sectional view of the diffuser passage 15 showing a firstembodiment. This diffuser passage 15 introduces from the diffuser entry14, the discharged air (indicated with white arrows in the drawing) fromthe outer circumferential end of the impeller 13, and allows the airflowguided to the passage between the shroud side wall surface 15 a and thehub side wall surface 15 b to flow out from a diffuser exit 18 into thescroll 16.

In the embodiment shown in the drawing, the hub side wall surface 15 bof the diffuser passage 15 is provided with an inclined plane 20 thatapproaches the direction of the shroud side wall surface 15 a in aposition on the downstream side in a portion parallel with a normal lineof the section of the impeller exit. This inclined plane 20 is aninclined area, formed on the hub side wall surface 15 b of the diffuserpassage 15, that becomes closer to the shroud side wall surface 15 a asthe distance from the diffuser entry 14 increases, approaching thediffuser exit 18 and narrowing an axial direction flow passage width W,which is a distance between the opposing wall surfaces, down to Wa.

That is to say, in the diffuser passage 15 of normal line direction flowpassage length L, the inclined area formed by the inclined wall surface20 inclines toward the direction of the shroud side wall surface 15 afrom the upstream side of a passage of a flow passage length La parallelwith the normal line direction, and is provided in a downstream portionof flow passage length Lb. Here, taking 1 as the flow passage length L,the preferred length of the downstream portion Lb, which becomes theinclined area, is a portion remaining on the downstream side where thelength La of the upstream portion parallel with normal line direction is0.3 to 0.7. In other words, the preferred position for providing theinclined area in the lengthwise direction of the diffuser passage 15 maybe set so that, where the flow passage length from the diffuser entry 14which is taken as a base point (0), to the diffuser exit 18, which istaken as an end point (1), is taken as L (L=1), the length La of theupstream portion is set at a ratio of 0.3 to 0.7, and the length Lb ofthe downstream portion is set at a ratio of 0.7 to 0.3. Therefore, sincethe flow passage length L is the total length of the length La of theupstream portion and the length Lb of the downstream portion (L=La+Lb),the total length L of the upstream portion length La and the downstreamportion length Lb is always “1”.

Moreover, it is preferable that the inclined area provided in thediffuser passage 15 be such that a projection amount of the inclinedwall surface from the hub side wall surface 15 b to the shroud side wallsurface 15 a is set to be approximately ⅓ to ⅕ of the passage width atthe diffuser exit 18, where the projection amount of the inclined wallsurface 20 reaches its maximum value Wb. That is to say, since themaximum value Wb of the projection amount is ⅓ to ⅕W (Wb≈⅓ to ⅕W), theaxial direction flow passage width Wa narrowed by the inclined wallsurface 20 is set to be approximately ⅔ to ⅘ of the axial direction flowpassage width W (Wa≈⅔ to ⅘W).

Moreover, as is the case with the inclined wall surface 20 mentionedabove, the angle of inclination θ where the inclined area is a planesurface, is preferably set less than or equal to 20 degrees based on thenormal line of the impeller exit section. However, a further preferableangle of inclination θ is greater than or equal to 2 degrees and lessthan or equal to 10 degrees based on the normal line of the impellerexit section.

The normal line and normal line direction of the impeller exit sectionmentioned above refers to a straight line or direction that extendsradially outward from an axial center line 17 of rotation of theimpeller 13 and passes the impeller exit section, and it practicallyapproximates the airflow direction.

As described above, the diffuser passage 15 provided with the inclinedarea formed by the inclined wall surface 20 is provided with: anupstream side area where the shroud side wall surface 15 a and the hubside wall surface 15 b are both parallel with the normal line directionand the axial direction flow passage width W is constant; and aninclined area on the downstream side where the axial direction flowpassage width W to the diffuser exit 18 side is narrowed by the inclinedwall surface 20, in which the hub side wall surface 15 b inclines towardthe shroud side wall surface 15 a.

Therefore the airflow introduced from the diffuser entry 14 is sloweddown by flowing through the diffuser passage 15, and recovers its staticpressure. However, at this time, on the downstream side closer to thediffuser exit 18, the airflow in a low velocity area, which occurs inthe vicinity of the wall surface and is thought to cause the airflow toseparate from the hub side wall surface 15 b, is guided to the inclinedwall surface 20 so that it gradually flows toward the shroud side wallsurface 15 a.

The low velocity area in this case is an area where a velocity componentin the radial direction from the diffuser entry 14 toward the diffuserexit 18 is low. In the example shown in the drawing, the radialdirection corresponds to the normal line direction.

As a result, the airflow in the low velocity area that has occurred inthe vicinity of the wall surface of the hub side wall surface 15 bincreases its velocity component in the radial direction. Therefore, inthe diffuser passage 15, the velocity distribution in the radialdirection becomes uniform and flow separation is unlikely to occurlocally.

As described above, if flow separation is unlikely to occur locally inthe diffuser passage 15, the surge flow rate can be reduced, enablingthe operation range of the centrifugal compressor to be widened. Inparticular, if the invention is applied in the case where a wide rangeof compressor operation is required from a small sized centrifugalcompressor such as a turbo charger provided with a vaneless diffuser,widening of the range can be easily achieved.

Incidentally, considering the case of providing a similar inclined areaon the shroud side wall surface 15 a, the axial direction flow passagewidth W becomes narrower in the opposite direction as it approaches thediffuser exit 18. However, since there is not a low velocity area whichis thought to be a cause of flow separation, present in close proximityto the wall surface of the shroud side wall surface 15 a in the vicinityof the diffuser exit 18, the airflow guided to the inclined wall surface20 is accelerated by gradually flowing toward the hub side wall surface15 b. Therefore, a difference between the accelerated velocity on theshroud side wall surface 15 and the velocity on the hub side wallsurface 15 b, in which the low velocity area is present, increases, andhence non-uniformity of the velocity distribution in the radialdirection increases.

Next, a first modified example of the inclined area mentioned above isdescribed, based on FIG. 2. The same reference symbols are given toparts that are the same as in the above embodiment, and detaileddescriptions thereof are omitted.

In this first modified example, in place of the inclined plane 20 inFIG. 1, an inclined curved surface 21 forms an inclined area. Thisinclined curved surface 21 is the same as the inclined plane 20 withrespect to the preferred position along the diffuser passage lengthdirection in which the inclined area is to be provided (ratio of lengthLb), and with respect to the maximum value Wb of the projection amountfrom the hub side wall surface 15 b toward the shroud side wall surface15 a, and the curvature may be appropriately set to satisfy theseconditions. The curved surface in this case may be either a concavecurved surface or convex curved surface when seen from inside thediffuser passage 15.

Since the radial direction velocity component of the low velocity areaairflow that has occurred in the vicinity of the wall surface of the hubside wall surface 15 b also increases even when the inclined curvedsurface 21 provided is formed by such an inclined area, the velocitydistribution in the radial direction is made uniform so that flowseparation becomes unlikely to occur locally.

Therefore, the surge flow rate can be reduced, and hence the operationrange of the centrifugal compressor can be widened and, in particular,if the invention is applied in a case where a wide range of compressoroperation is required of a small size centrifugal compressor providedwith a vaneless diffuser, the compressor operation range can be easilywidened.

Next, a second modified example of the inclined area mentioned above isdescribed, based on FIG. 3. The same reference symbols are given toparts that are the same as in the above embodiment, and detaileddescriptions thereof are omitted.

In this second modified example, in place of the inclined plane 20 inFIG. 1, an inclined inflected line 22 forms an inclined area. Thisinclined inflected line 22 is formed from a plane inclined section 22 aand a parallel section 22 b on the diffuser exit 18 side, the parallelsection 22 b in this case being parallel to the shroud side wall surface15 a and the hub side wall surface 15 b.

Moreover, this inclined inflected line 22 is the same as the inclinedplane 20 with respect to the preferred position along the diffuserpassage length direction in which the inclined area is to be provided(ratio of length Lb), and with respect to the maximum value Wb of theprojection amount from the hub side wall surface 15 b toward the shroudside wall surface 15 a.

Since the radial direction velocity component of the low velocity areaairflow that has occurred in the vicinity of the wall surface of the hubside wall surface 15 b also increases even when the inclined inflectedline 22 provided is formed by such an inclined area, the velocitydistribution in the radial direction is made uniform so that flowseparation becomes unlikely to occur locally.

Therefore, the surge flow rate can be reduced, and hence the operationrange of the centrifugal compressor can be widened and, in particular,if the invention is applied in a case where a wide range of compressoroperation is required of a small size centrifugal compressor providedwith a vaneless diffuser, the compressor operation range can be easilywidened.

The inclined inflected line 22 shown in the drawing is a combination ofthe inclined section 22 a and the parallel section 22 b. However, aninclined section 22 a of two or more steps may be combined, andfurthermore, a curved surface may also be combined.

FIG. 4 is a sectional view showing a second embodiment of thecentrifugal compressor according to the present invention. The samereference symbols are given to parts that are the same as in the firstembodiment, and detailed descriptions thereof are omitted.

In the present embodiment, a diffuser passage 30 is divided into threeareas. Specifically, from the upstream side, a hub side inclined section31, a parallel section 32 parallel with the normal line direction of theimpeller exit section, and a shroud side inclined section 33 areintegrally connected. Therefore, compared to the first embodiment shownin FIG. 1, the hub side inclined section 31 is added on the mostupstream side, and, in the shroud side inclined section 33, the shroudside wall surface 30 a and the hub side wall surface 30 b are arrangedparallel with each other so as to incline toward the shroud side at thesame inclination angle.

When employing this kind of configuration also, in the shroud sideinclined section 33 of the diffuser passage 30, the hub side wallsurface 30 b is provided with an inclined area that approaches theshroud side in a position on the downstream side of the parallel section32, which is a portion parallel with the normal line of the impellerexit section. That is to say, by having the hub side wall surface 30 bof the shroud side inclined section 33 approach the shroud side, aninclined area having the same effect as that of the inclined plane 20mentioned above is formed.

As a result, since radial direction velocity component of the lowvelocity area airflow that has occurred in the vicinity of the wallsurface of the hub side wall surface 30 b is increased by the hub sidewall surface 30 b that serves as an inclined plane similar to theinclined plane 20, the velocity distribution in the radial direction ismade uniform so that flow separation becomes unlikely to occur locallyin the diffuser passage 30.

Therefore, the surge flow rate can be reduced, and hence the operationrange of the centrifugal compressor can be widened and, in particular,if the invention is applied in a case where a wide range of compressoroperation is required of a small size centrifugal compressor providedwith a vaneless diffuser, the compressor operation range can be easilywidened.

As described above, on the hub side wall surface 15 a of the diffuserpassage 15, since the inclined area that approaches the shroud side isprovided in a position on the downstream side of the portion parallelwith the normal line direction of the impeller exit section, the radialdirection velocity of the low velocity area that occurs on the hub sidewall surface 15 a increases, and the radial direction velocitydistribution within the diffuser passage 15 is made uniform. Therefore,airflow is unlikely to separate locally from the wall surface in thevicinity of the diffuser exit 18 of the diffuser passage 15, and hencethe surge flow rate can be reduced and the compressor operation rangecan be widened.

The present invention is not limited to the above embodiments and can beappropriately modified without departing from the scope of the presentinvention.

1. A centrifugal compressor provided with a diffuser passage forrecovering static pressure by slowing down discharged air from an outercircumferential end of an impeller that rotates within a housing,wherein a hub side wall surface of said diffuser passage is providedwith an inclined area that approaches toward a shroud side, in aposition on a downstream side of a portion parallel with a normal linedirection of a section surface of an impeller exit.